Not Applicable
Not Applicable
The present invention is directed to the recovery of elemental sulfur from hydrogen sulfide containing gas streams. More specifically, the present invention is directed to improvements in a Claus sulfur plant using oxygen enrichment and recycle of gases using an ejector to increase capacity and moderate flame temperatures in the reaction furnace.
It is known in the prior art to recover elemental sulfur from hydrogen sulfide containing gas streams as is set forth in the article xe2x80x9cFundamentals of Sulfur Recovery by the Claus Processxe2x80x9d by B. Gene Goar, published in the 1977 Gas Conditioning Conference Report.
It is also known to use oxygen enrichment in the operation of a Claus sulfur plant in order to increase the capacity of hydrogen sulfide handled as well as the total throughput of the plant as set forth in the article xe2x80x9cOxygen Use in Claus Sulfur Plantsxe2x80x9d by M. R. Gray and W. Y. Svrcek published in the 1981 Gas Conditioning Conference Report. In that article it was disclosed that oxygen can be added to the air feed to the burner of the reaction furnace in order to increase the amount of hydrogen sulfide which is combusted to sulfur dioxide for later catalytic conversion with additional hydrogen sulfide to the elemental liquid sulfur product of the Claus process. The article recites that the maximum capacity increase which can be achieved with oxygen enrichment is determined by the pressure drop through the plant and the reactor space velocities. However, a further limitation set forth in the article is that for a given plant stream, temperatures and sulfur condenser capacity may limit the potential capacity increase using oxygen enrichment. Specifically, stream temperatures in the reaction furnace and in the converter beds may increase due to oxygen enrichment and in fact such increase from oxygen enrichment reaches the maximum tolerable temperature of the materials used in such a furnace, namely the refractory lining.
In the 1983 publication by Linde of Union Carbide entitled xe2x80x9cClaus Plant Oxygen Enrichmentxe2x80x9d, it is noted that oxygen-enrichment limitations exist for rich H2S streams due to temperature limits in the furnace or waste heat boiler of a Claus plant.
U.S. Pat. No. 3,681,024 discloses the addition of oxygen and a recycle gas to a Claus furnace where the recycle gas is delivered by a blower.
U.S. Pat. No. 3,822,341 describes a Claus plant using oxygen enrichment in which one source of oxygen is initially used to strip residual SO2 from a sidestream in vessel 92 before the oxygen stream in line 96 is optionally recycled to be included with the oxygen in line 12 going to the combustion zone of the waste heat boiler 8. As recited at col. 5, lines 65-68 of the specification. Because the oxygen content of such a stream is completely consumed in an exothermic reaction this stream can not be utilized as a moderating medium for the flame temperature of the reaction furnace. As described by Goar, Claus sulfur plants typically have an adiabatic reaction furnace followed by a waste heat boiler. The excessive temperature problem with oxygen enriched operation occurs in the adiabatic reaction furnace. U.S. Pat. No. 3,822,341 ignores the existence of this problem.
U.S. Pat. No. 4,153,674 discloses a Claus plant and tailgas cleanup plant wherein a gas stream in line 20 is removed from the tailgas system and is returned or recycled to the front end of the Claus plant 7. This patent does not consider oxygen enrichment or flame temperature moderation by a recycle stream. Also the tail gas is reacted to convert all sulfur to hydrogen sulfide which is absorbed, stripped and returned to the Claus plant.
U.S. Pat. No. 4,552,747 is a process for recycle moderation of the high temperatures in the Claus furnace induced by oxygen-enrichment by recycling an effluent stream from the first condenser. The recycle uses a mechanical blower.
U.S. Pat. No. 4,526,590 describes a sulfur recovery process with separate sulfur and water condensation.
U.S. Pat. No. 4,756,900 discloses a process for splitting the effluent from the waste heat boiler of a Claus furnace and recycling a portion thereof using a separate sulfur condenser and a mechanical blower to moderate the high furnace temperatures induced by oxygen enrichment.
The present invention overcomes the shortcomings of the prior art by increasing throughput of a Claus plant by oxygen enrichment to an extent beyond that contemplated in the prior art due to flame temperature limitations using a safer method and less mechanical and capital resources. This is achieved by the recycle of an intermediate gas stream from the first condenser in the Claus train to moderate the temperature in the flame of the burner of the reaction furnace using an ejector.
The present invention is directed to a process for recovering sulfur from a feed gas stream rich in hydrogen sulfide wherein the gas stream is partially combusted with an oxygen-enriched gas in a Claus reaction furnace, the combustion effluent is cooled with the attendant condensation and separation of sulfur in the first condensation zone and the remaining effluent stream is passed through at least one stage of reheating, conversion in a catalytic Claus reaction zone and cooling with attendant condensation and separation of sulfur in an additional condensation zone, recycling a portion of the combustion effluent to the reaction furnace zone to moderate the temperature of the reaction furnace zone wherein the improvement comprises performing the recycle with the motive power of a high pressure stream in an ejector where the recycle stream is the suction stream to the ejector.
Typically the process uses three stages of reheating, conversion and cooling and separation subsequent to the first condensation zone.
The process is relevant for hydrogen sulfide containing streams wherein the sulfide is in the range of 60 to 100 mole %. Preferably the hydrogen sulfide content of the feed gas is 80 to 100 mole %.
Preferably the oxygen enrichment of the reaction furnace is in the range of 32 to 100 mole %, more specifically 40-75 mole %.
The recycle stream flow rate should be in the range of 5 to 60% of the combustion effluent stream from the first condensation zone.
Preferably the temperature of the reaction furnace zone is maintained in the range of 2400 to 2800xc2x0 F.
The invention also is directed to a system for recovering sulfur from a feed gas stream rich in hydrogen sulfide by the Claus reaction including a reaction furnace for partially combusting the feed gas stream with an oxygen enriched gas, a first condensing means for cooling and condensing sulfur from the combustion effluent, at least one train comprising a reheater means, a catalytic Claus reactor and an additional condensing means, means for rewarming, further reacting and recovering sulfur from said effluent, recycle means for recycling a portion of the combustion effluent to said reaction furnace wherein the improvement comprising the recycle means is an ejector.
Preferably the system has a recycle means constituting a conduit just downstream of the first condenser for recycling a portion of the combustion effluent gas through the conduit and delivered by a recycle ejector through a subsequent conduit to the reaction furnace of the Claus plant system.